Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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30 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM
COATINGS & LININGS
T
A cermet is a metallic matrix with dis-
persed ceramic particles. Tungsten
carbide dispersed in nickel (WC-Ni) is
a cermet. This article discusses WC-
Ni brazing on the surface of an impel-
ler to improve wear and corrosion re-
sistance. The coating was applied in a
conventional vacuum furnace while
rotating the impeller to develop a
homogeneous and uniform coating
layer during the brazing process. Ap-
propriate revolutions per minute were
derived using the finite element
method with input on centrifugal and
gravity forces. The service life of the
impeller with the WC-Ni brazed coat-
ing was measured to be almost eight
times longer than the life of the non-
brazed impeller.
The application of surface treatments—
coating layers with properties superior to
those of the substrate—generally are used
to improve the service life of equipment in
i n d u st r i a l s e tt i n g s .
1 - 2
C o n t r o l l i n g t h e
surface layer's mechanical properties is
considered to be an important attribute for
current engineering applications. Minor
damage of the surface sometimes results in
mechanical component failure, which has
caused fatal accidents. Surface treatments
can also provide a longer service life with a
consequent reduction of total costs.
3
WC-based composite layer coatings
have good resistance to abrasive and ero-
sive wear due to the presence of WC parti-
cles that are dispersed in an alloy binder
such as nickel or cobalt.
4
Thermal spraying,
such as high-velocity oxygen f lame spray-
ing, plasma spraying, wire arc spraying,
flame spraying, and other techniques,
5
is a
useful technology to produce WC-based
composite layer coatings. Because it is a
mechanically bonded coating, however,
thermal spraying has low bond strength
between the coating layer and the sub-
strate. Also, it exposes the WC powder to
oxygen or water vapor, w hich leads to
porosity and oxide formation.
3
Another process for preparing a WC
composite layer coating is based on the infil-
tration brazing technology. In brazing, the
substrate is wetted by the melted filler metal
and a metallurgical bond is formed. The
bond strength is superior to that of thermal
sprayed coatings.
6
It can be applied to
curved, inner, or outer surfaces simultane-
ously, which can reduce the cost of machin-
ing the coating surface after brazing. It also
has less distortion, lower residual stresses,
and lower substrate dissolution.
7
Tungsten carbide dispersed in nickel
(WC-Ni) is a cermet, which is a metallic
matrix with dispersed ceramic particles,
and is widely used in industries due to its
high hardness and good wear resistance.
Gao, et al.
8
reported on a WC-Ni brazed
coating made with WC and Ni-based metal
powders applied through high-temperature
vacuum brazing. Lu and Kwon
6
reported
that wear resistance of the WC-Ni brazed
coating is better than other overlaid coat-
ings under wet sand abrasive wear and
rotating slurry erosive wear. Klaasen, et al.
9
investigated th e strength and residual
stress between a WC-Ni brazed coating and
the substrate. They found that the plastic-
ity and proof stress of the filler metal affect
the reliability of WC-Ni brazed coating.
Development of WC-Ni Cermet
Brazing Process
Hyung g oun Joo, School of Mechanical
Engineering, Yonsei University, Seoul,
South Korea
Kang y ong Lee, State Key Laboratory of
Structural Analysis for Industrial
Equipment, Department of Engineering
Mechanics, Dalian University of
Technology, Liaoning, People's
Republic of China
g uo Ming Luo and d a Quan Z H ang,
Department of Environmental
Engineering, Shanghai University of
Electric Power, Shanghai, People's
Republic of China